It is presumed that the mechanosensory machinery in the stereocilia bundles of the inner ear hair cells is very vulnerable to intense mechanical stimulation. Yet, we still do not know what exactly is damaged within the transduction machinery and whether these damages can be repaired. However, it may represent a significant component in noise-induced hearing loss. The ability of the hair cell to repair these damages may determine whether the noise-induced temporary shift of hearing thresholds would recover or progress into permanent hearing loss. Interestingly, damage to the actin core of stereocilia has been known as a hallmark of permanent noise-induced hearing loss for more than two decades. However, little is known on how this damage develops or repairs. We have recently initiated a systematic study of the effects of mechanical overstimulation on the mechano-electrical transduction and the ultrastructure of stereocilia bundle in young postnatal cochlear hair cells. An anticipated effect of overstimulation is the loss of tip links that are known to regenerate. Using a novel immunogold scanning electron microscopy technique, we have discovered that the tip link regeneration may involve a two-step molecular remodeling that includes formation of the temporary links with protocadherin-15 at both ends of the link and then replacing them with the more mature links containing protocadherin-15 and cadherin-23. Specific Aim 1 of the proposed project will explore regeneration of the tip links in mechanically damaged cochlear inner hair cells and determine whether molecular remodeling occurs in regenerating tip links. Our preliminary data also show that mechanical overstimulation does not abolish all tip links in the cochlear hair cells; many of them withstand overstimulation. However, the remaining mechanotransducer current loses slow adaptation and has no evidence of the tension within the transduction machinery that is normally present at rest. Specific Aim 2 will explore the mechanisms of re-tensioning of the transduction apparatus in mechanically damaged cochlear inner hair cells. Investigating Ca2+ entry into individual stereocilia, we found that the cochlear inner hair cells regulate resting tension within the transduction apparatus via a novel, extremely slow Ca2+-dependent process. This regulation is likely to be affected in mechanically damaged hair cells. Specific Aim 3 will determine the mechanisms of this slow regulation and its potential disruption by intense deflections of the hair bundle. Finally, we foun that overstimulation causes small, nanometer-scale gaps in the actin core of stereocilia, similar to the ones that we previously observed in degenerating stereocilia lacking gamma-actin. Specific Aim 4 will explore whether the hair cells can repair these gaps and whether gap formation or repair depends on gamma-actin. We believe that our study is a long-awaited step in exploring the potential mechanisms of the noise-induced hearing loss.